In recent years, techniques for converting natural energy, such as sunlight or wind power, into electrical energy have been receiving attention. Furthermore, there has been an increased demand for non-aqueous electrolyte secondary batteries as batteries having a high energy density capable of storing a large amount of electrical energy. Among non-aqueous electrolyte secondary batteries, lithium ion secondary batteries are promising in terms of being light weight and having high electromotive force. However, lithium ion secondary batteries have a drawback in that they have low heat resistance because of the use of an organic solvent as the electrolyte component. Furthermore, with the growing market of non-aqueous electrolyte secondary batteries, the prices of lithium resources are rising.
Accordingly, development has been conducted on molten-salt batteries which use an incombustible molten salt as an electrolyte. Molten salts have excellent thermal stability, in which safety can be secured relatively easily, and are suitable for continuous use under high-temperature range conditions. Furthermore, since molten-salt batteries can use, as an electrolyte, a molten salt containing, as a cation, an inexpensive alkali metal other than lithium (in particular, sodium), the production costs thereof are low.
A molten-salt battery is a general term for any battery that contains a salt in a molten state (molten salt) as an electrolyte. The salt used as an electrolyte of the molten-salt battery is a liquid having ion conductivity (ionic liquid) at least in the molten state.
In a positive electrode of a sodium molten-salt battery, a sodium-containing transition metal oxide, such as sodium chromite, is used as a positive electrode active material. Furthermore, in a negative electrode, as a negative electrode active material, for example, sodium, a sodium alloy, a metal that alloys with sodium, a carbon material, a ceramic material, or the like is used. The positive electrode and the negative electrode can each be formed, for example, by using a mixture containing such an active material and a binder.
PTL 1 discloses that polyvinylidene fluoride (PVDF) is used as a binder of an electrode for a molten-salt battery.
Fluororesins, such as PVDF, have a high binding property and a good handling property, and therefore, are used as an electrode binder not only for a molten-salt battery, but also for an organic electrolyte secondary battery, such as a lithium ion secondary battery or sodium ion secondary battery (for example, refer to PTL 2).